The use of overhead projectors for illustration and supplementing lectures and instructional material has become widespread in recent years owing to the ease with which the material for projection may be prepared, and the ready availability at relatively low cost of projection equipment. The projector normally consists of a light box which casts a converging beam of light into a prism-like element, which, in turn, casts a light beam upon a projection screen. In use, the material being projected is positioned upon the upper surface of a light transmitting wall forming part of the light box for support and illumination. Much of the currently used material is provided by running a specialized type of light-transmitting sheet material through electrostatic type copiers to obtain flexible but durable copies which can be used many times, and which are of standardized overall dimensions.
One of the problems encountered when using such transparencies is the tendency to slide upon the illuminated surface of the light box during projection, creating a disturbing effect upon the audience watching the display. Another problem lies in the fact that such copies reproduce material in essentially black and white, the continued use of which tends to become tiring.
It is known in the art to provide transparency positioning means for locating the center of the transparency in coincidence with the optical projection axis of the projector, but such structure precludes the use of color added to the projection system.
In my above mentioned prior application, there is disclosed an improved frame construction, the present disclosure relating to a further simplification of structure for accomplishing the same end.